1. Field of the Invention
The present invention relates to a PWM (pulse-width modulation) motor driving device for driving a brushless DC (direct-current) motor, such as a capstan motor employed in a VCR (videocassette recorder), while controlling the rotation rate thereof by a PWM-based method.
2. Description of the Prior Art
FIG. 15 shows an example of the circuit configuration of the output stage of a motor driving device. In this figure, TUU, TVU, TWU, TUL, TVL, and TWL represent NPN-type power transistors. LU, LV, and LW represent the coils of a motor for the U-, V-, and W-phases, respectively, that are connected in a Y-shaped connection. The transistors TUU, TVU, and TWU receive at their collectors a motor drive voltage VM, and the transistors TUL, TVL, and TWL have their emitters connected to ground GND. The emitter of the transistor TUU and the collector of the transistor TUL are connected to the U-phase coil LU of the motor, the emitter of the transistor TVU and the collector of the transistor TVL are connected to the V-phase coil LV of the motor, and the emitter of the transistor TWU and the collector of the transistor TWL are connected to the W-phase coil of the motor.
For example, when, as indicated by an arrow Y1 in FIG. 15, a current is passed from the U-phase coil LU of the motor to the V-phase coil LV thereof, the upper output transistor TUU and the lower output transistor TVL are turned ON. Here, in a PWM motor driving device, those transistors are, instead of one or both of them being kept ON, switched between ON and OFF at a predetermined frequency (hereinafter this will be called PWM switching). Thus, on the basis of the ON/OFF duty factor achieved by such PWM switching, the amount of electric power supplied to the motor, and thus the rotation rate thereof, is controlled.
In a device that drives a motor linearly, to prevent the upper output transistors from being saturated, the base currents thereof are controlled according to the highest among the U-phase voltage (the voltage at the node PU), the V-phase voltage (the voltage at the node PV), and the W-phase voltage (the voltage at the node PW); similarly, to prevent the lower output transistors from being saturated, the base currents thereof are controlled according to the lowest among the U-phase voltage, the V-phase voltage, and the W-phase voltage.
Here, for example, when the upper output transistor TUU turns from ON to OFF, the back electromotive force appearing in the coil LU causes, as indicated by an arrow Y2 in FIG. 15, a current to flow through a diode D parasitic on the lower output transistor TUL, and thereby causes the U-phase voltage to become lower than the ground level by the voltage drop across the diode D. As a result, whereas ideally the lowest voltage should appear in, among the U-, V-, and W-phases, the phase of whichever coil is currently being driven by the lower output transistors, actually a voltage lower than that appears in the phases of the other coils that are currently not being driven by the lower output transistors.
For this reason, in a PWM motor driving device, while the upper output transistors are undergoing PWM switching, if prevention of saturation of the output transistors is attempted by the method described above, the controlling of the base currents of the lower output transistors is attempted according to the voltage of the phase of the coil that is currently being driven by the upper output transistors, and therefore the lower output transistors cannot be properly prevented from saturation. Similarly, when the lower output transistors are undergoing PWM switching, the upper output transistors cannot be properly prevented from saturation.
Moreover, in a driving method based on PWM, since the coils of the motor are driven with pulses as described above, whereas good motor rotation efficiency can be achieved, smooth motor rotation cannot be obtained because the waveform of the currents obtained to be passed through the coils of the motor is not sinusoidal.
An object of the present invention is to provide a PWM motor driving device that achieves enhanced motor rotation characteristics by preventing saturation of output transistors.
Another object of the present invention is to provide a PWM motor driving device that produces less ripples in motor rotation.
To achieve the above objects, according to one aspect of the present invention, a PWM motor driving device that controls the amount of electric power supplied to a motor by controlling the ON/OFF duty factor of an output transistor connected between a power source and the motor is provided with: phase detecting means for detecting, among a plurality of phases of the motor, the phase which is currently being driven by the output transistor; and saturation preventing means for performing control according to the voltage of the phase detected by the phase detecting means in such a way that the output transistor is not saturated.
This circuit configuration permits the output transistors to be controlled so as not to be saturated according to the voltage of the phase in which the motor is currently being driven. Thus, for example, when the upper output transistors are undergoing PWM switching, the lower output transistors are controlled so as not to be saturated no longer according to the voltage of the phase in which the motor is currently being driven by the upper output transistors as practiced conventionally. This helps obtain better motor rotation characteristics.
According to another aspect of the present invention, a PWM motor driving device that controls the amount of electric power supplied to a motor by controlling the ON/OFF duty factor of a switching device connected between a power source and the motor is provided with: ripple cancel signal generating means for generating, based on a signal representing the rotational position of the motor, a ripple cancel signal having a triangular waveform with maxima thereof synchronized with minima in torque; and duty factor controlling means for controlling the ON/OFF duty factor of the switching device according to a signal obtained by adding the torque control signal and the ripple cancel signal together.
This circuit configuration permits the ON/OFF duty factor, achieved by PWM switching, of the output transistors to be controlled according to a signal having a triangular waveform with maxima thereof synchronized with minima in torque. This makes it possible to pass more current at times of low torque and thereby achieve smoother motor rotation.